Feature addition: Backtracking Line Search for optimization

[NeelGhoshal]

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Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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What to do if you already signed the CLA

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[NeelGhoshal]

@googlebot I signed it!

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[NeelGhoshal]

For Issue#1414

[kyscg]

Suggested change

	alpha = 1):
	alpha = 0.3):

Doesn't the Backtracking Line Search algorithm have a constraint that $\alpha$ be between 0 and 0.5?
Stanford EE64a Slides, see page 6